The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
This invention relates in general to a sealing apparatus. More particularly, this invention relates to a finger seal apparatus which is adapted to be interposed between relatively high and low pressure cavities to provide sealing along a rotating member.
Gas turbine engines can employ sealing devices in various capacities where it is necessary to restrict the flow of fluid or gases from one portion of the engine to another. A common use is for separating the primary engine flowpath from the secondary flowpath. The primary engine flowpath directs the flow of gases to the compressor and turbine stages, from which the engine derives thrust or power. The secondary flowpath comprises a series of conduits for delivering compressed air throughout the engine for performing a variety of functions. Compressed air can be used, for example, to cool individual components, provide a bleed air source, buffer the lubricated bearing cavities, control the ventilation among engine cavities and structures, and affect the thrust balance of the engine. Loss of compressed air from the secondary flowpath through leakage can have a substantial adverse effect on the performance of these functions. In a turbine engine at least one sealing device is typically required for each turbine and compressor stage of the engine.
Another common use for sealing devices in turbine engines is for separating the secondary flowpath from engine cavities containing fluids such as lubricating oil. In pressurized aircraft, bleed air taken from the secondary flowpath supplies the aircraft environmental control system. Even small amounts of oil in the bleed air can render it unsuitable for this purpose. Further, oil leakage can lead to coking of the seal, and ultimately reduced seal life. To prevent this, buffered sealing devices typically are incorporated adjacent lubricated bearings and engine oil sumps.
Labyrinth seals are noncontacting seals and are commonly used in gas turbine engines to seal leakage flows along a shaft. In labyrinth seals the clearance is fixed and must be large enough to accommodate centrifugal and thermal growth of the shaft, thermal changes in the surrounding structure, and dynamic motion of the shaft to avoid contact between the seal and the rotor. Radial excursions can be substantial, particularly in propulsion gas turbine engines used in aircraft. Brush seals and finger seals are two types of compliant contacting seals that can reduce seal leakage over typical labyrinth seals.
Brush seals are typically comprised of a ring-shaped pack of small diameter wire bristles set at an angle to the radial direction and sandwiched between a front and back washer. Because the bristles are set at an angle, they can act like cantilevered beams and bend out of the way during shaft perturbations.
Finger seals are typically comprised of a plurality of flexible fingers fixed at one end, and the opposite ends sealingly engaging a surface that is rotatable relative thereto. The fingers of a finger seal can be an integral part of the seal, usually formed by machining a series of curved slots in a forged ring or a length of sheet stock. The slots can be of consistent length and extend from a common edge of the material. A complete finger seal can be made up of two or more layers of fingers relatively positioned adjacent each other.
A problem with finger seals is that at high shaft speeds, the rubbing contact between the fingers and the rotating surface can cause excessive heating of the components, which in turn lowers the durability and ultimately limits the life of the seal. The problem is further aggravated when the seal is used in high temperature applications. Accordingly, the desired sealing is undermined leading to engine inefficiencies. As a result, various techniques have been employed to increase the speeds and temperatures at which finger seals may be effectively used. A technique commonly employed for increasing durability is to provide a hardened and very smooth coating on the contacting surface of the rotating component.
Thus, there exists a need for a finger seal capable of extended use in high speed and temperature environments, such as those involving gas turbine engines.
It is an object of an exemplary form of the present invention to provide a finger seal apparatus capable of extended use in high speed and high temperature environments.
It is a further object of an exemplary form of the present invention to provide a finger seal capable of extended use in gas turbine engines.
It is a further object of an exemplary form of the present invention to provide a finger seal that can be interposed between relatively high and low pressure cavities.
It is a further object of an exemplary form of the present invention to provide a finger seal that can be interposed between relatively high and low pressure cavities to provide sealing along a rotating member.
It is a further object of an exemplary form of the present invention to provide a finger seal that can provide sealing between a rotatable member and a housing circumscribing the rotatable member.
It is a further object of an exemplary form of the present invention to provide a finger seal that can provide sealing between a rotatable turbine shaft and a housing circumscribing the rotatable turbine shaft.
It is a further object of an exemplary form of the present invention to provide a finger seal that can be interposed between an upstream region of relatively high fluid pressure and a downstream region of relatively lower fluid pressure to inhibit fluid flow therebetween.
It is a further object of an exemplary form of the present invention to provide an annular sealing apparatus capable of being placed for disposition between a body defining a bore and a rotating member, such as a seal rotor mounted on a rotating shaft or the shaft itself, rotatably received in the bore to inhibit fluid leakage therebetween.
It is a further object of an exemplary form of the present invention to provide an improved noncontacting finger seal that includes lift pads attached to downstream fingers, upstream fingers that do not contact the rotor, and pressure balance features.
Further objects of exemplary forms of the present invention will be made apparent in the following Best Mode for Carrying Out Invention and the appended claims.
The foregoing objects are accomplished in exemplary forms of the present invention by a finger seal apparatus. The apparatus can comprise axially juxtaposed first and second comb-like members or finger elements. Each finger element can have a plurality of uniformly spaced and integral fingers. The fingers can circumscribe a rotating member. Each finger can have substantially the same width and the same spiral curvature, and be separated from an adjacent finger by a uniform gap. First and second finger elements can be adjacently positioned so that the fingers of each block the gaps in the other.
At the innermost end of each finger of the first finger element is a lift pad operative to provide sealing to the rotating member in a noncontacting manner. Each lift pad can be substantially larger than its respective finger and shaped to enhance hydrodynamic lifting so as to enable at least a portion of the lift pad to lift away from the rotating member and run on a thin film of fluid upon sufficient rotational speed of the rotating member.
In an exemplary form of the invention an annular finger seal apparatus can be interposed between an upstream region of relatively high fluid pressure and a downstream region of relatively lower fluid pressure to provide noncontact sealing along a rotatable member. In the exemplary form only the fingers of the first finger element have a lift pad. The first finger element is positioned downstream of the second finger element. Each lift pad extends from its respective finger in a downstream direction. The innermost diameter of the fingers of the second (or upstream) finger element is larger than the innermost diameter of the lift pads. The innermost diameter of the fingers of the second finger element is also smaller than the outermost diameter of the lift pads. In positioning of the finger seal apparatus, each lift pad can be arranged adjacent to the rotating member while each finger of the second finger element is spaced from the rotatable member. At least a portion of each lift pad is operative to lift away from the rotating member upon at least one of sufficient rotational speed of the rotating member or hydrostatic pressure loading adjacent the lift pad. In an exemplary form of the invention, upon sufficient rotational speed of the rotating member, each lift pad can ride on a thin film of fluid intermediate the rotating member and the lift pad.